How do I construct a call to contract return string using inline assembly?

Question

pragma solidity ^0.4.21;
contract Test1 {
    function add(int a, int b) returns(string){  //Simply add the two arguments and return
    return "Qmrwokdkeoiudkeooiskelpdislpeosj;sd[opek;s[pkfe,;soeipsoekf;sodfpsoe,mfoskfesktgfetg";
}
}
contract Test2 {
Test1 test1;

function Test2(){  //Constructor function
    test1 = new Test1();  //Create new "Test1" function
}

function test(string a, string b) constant returns (string c){
    address addr = address(test1);  //Place the test1 address on the stack
     bytes4 sig = bytes4(sha3("add(int256,int256)")); //Function signature

   assembly {
    let x := mload(0x40)   //Find empty storage location using "free memory pointer"
    mstore(x,sig) //Place signature at begining of empty storage 
    mstore(add(x,0x04),a) //Place first argument directly next to signature
    mstore(add(x,0x24),b) //Place second argument next to first, padded to 32 bytes

    let success := call(      //This is the critical change (Pop the top stack value)
                        5000, //5k gas
                        addr, //To addr
                        0,    //No value
                        x,    //Inputs are stored at location x
                        0x44, //Inputs are 68 bytes long
                        x,    //Store output over input (saves space)
                        0x20) //Outputs are 32 bytes long

    c := mload(x) //Assign output value to c
    mstore(0x40,add(x,0x44)) // Set storage pointer to empty space
}
 }
}

but c is empty string

Answer 1

I see at least one mistake:

When you declare string c as a return value, it's actually string memory c. You need to assign the memory pointer of the start of the string to c. I think in your case that's just x:

c := x //Assign output value to c